Well because I had got my original carier sim yesterday and wanted a few ios4 apps to install which required post notification certificates
If you install
Whited00r 6 Normal version for iPhone 3G which is especially customized for Factory Unlocked iPhone's or for users like you that have the original carrier SIM then when connecting to iTunes it will activate the iPhone & also install a carrier bundle & also install a set of
Push Certificates. Besides your iPhone 3G will be
automatically jailbroken with Cydia already installed & given access to the free apps of ATM.

Now because Whited00r 6 is based on iOS 3.1.3 but tweaked, optimized & with added features then apps that requires iOS 4 or newer will not work.
OTOH there are more than 550 free compatible apps in
App Time Machine (ATM) which is included in Whited00r 6 so if you tell me which iOS 4 apps you want, I can search ATM to see if there are versions of those apps available for WD 6.
I spoke with Brian (bcooperizcool) yesterday and he told me that the iOS 4.1 Pre-Jailbroken firmware linked in post #1 has some bugs & causes some problems.

Maybe that is why Soltmeal broke the download link.
Also, I posted explanations & a f0rcast screen capture that shows that this firmware fakes that it is iOS
4.3.2 so it will let you install apps that are not compatible with it & that will never work as well as letting Cydia download & install incompatible apps & tweaks causing multiple problems.

If you absolutely want iOS 4, I suggest you download from & install with iTunes the
stock iOS 4.2.1, activate it with your SIM, then jailbreak it using redsn0w_win_0.9.14b2 or for Mac: redsn0w_mac_0.9.14b2.
Then you could install Brian's
Archetype which is great for tweaking & optimizing many iOS firmware versions or whatever other Cydia tweaks you want.
*****
Since you say that you have the original SIM from the original carrier and obviously the iPhone carrier locked contract is finished, I suggest you check the FAQ page of the carrier as many will
Factory Unlock the old 3G for free.
I did this recently for a friend's iPhone 4S before he came to visit me in Costa Rica so he could use a local much cheaper prepaid SIM instead of paying very expensive AT&T roaming charge. It took about 3 days for AT&T's support to do it and then simply connecting the iPhone 4S to iTunes factory unlocked it.
The advantage is you can then use any SIM from any carrier which is useful if traveling but also you will get a better re-sale price if you eBay it. Also the factory unlock is permanent so you can flash any firmware & use any SIM to activate it.